1. Field
The following disclosure is generally concerned with mid-IR laser systems based upon quantum well semiconductors, and specifically concerned with short external cavity, highly compact quantum well lasers.
2. Related Technology
Unipolar Quantum well lasers, sometimes referred to by a name coined by early pioneers “quantum cascade lasers” or QCLs, were first suggested in the 70s and finally reduced to practice in a laboratory late in the 80s. These lasers enjoy remarkably unique properties quite unlike other semiconductor laser systems. Quite unlike their predecessor cousins the diode laser, these lasers are formed from a single semiconductor type (either N-type or P-type) and do not include lasing at a band gap of a semiconductor junction. These devices which lase in the valuable mid-IR spectrum can be fashioned to support extremely wide gain bandwidths, they are suitable for high-power output, they are very small, inexpensive, efficient and durable. Despite these, quantum well lasers have not yet made an appreciable commercial impact. Their use remains almost exclusively restricted to professional research laboratories having highly specialized supporting equipment. Some versions of the systems require complex cooling apparatus, sophisticated electronic drive and detection means and other specialized optical support. Recently, quantum well gain systems have been arranged in conjunction with optical resonators which include a free space portion. Sometimes called in the art “external cavity QCLs” or “ECQCLs”, these arrangements permit valuable access to the resonator cavity which was not otherwise available in devices with gain medium having integrated end mirrors. By way of the cavity access, advanced wavelength tuning schemes are just being suggested at the time of this writing.
QCLs have been described in extensive patent publications from an early day as pioneers recognized their immense future value. In particular, Bell Laboratories now Lucent Technologies, produced at least the following patents related to early quantum cascade laser systems. These include U.S. Pat. Nos. 5,311,009; 5,457,709; 5,502,787; 5,509,025; 5,570,386; 5,727,010; 5,745,516; 5,901,168; 5,936,989; 5,978,397; 6,023,482; 6,055,254; 6,055,257; 6,091,753; 6,134,257; 6,137,817; 6,144,681; and 6,148,012. Of course, many others also have since made interesting inventions around the QCL foundation.
Of particular importance for this disclosure is quantum well lasers configured in external cavity configurations.
Recently, new patent publications have just started to suggest these combinations. In particular, US Patent Application Publication 2003/0043877, titled “multiple wavelength broad bandwidth optically pumped semiconductor laser” teaches quantum well based system having tunability taken up outside the gain medium. Inventor Kaspi further suggests an optically pumped version of this systems which requires special optical coupling between a pump source and the gain medium device—i.e. addition ex-cavity cooperation.
Another important patent related publication is teaching by Masselink et al published Sep. 29, 2005 as patent application publication numbered US 2005/0213627. This invention includes a quantum cascade laser structure coupled to an external cavity to effect wavelength tuning. Masselink et al systems are particularly distinguished in that they employ a mechanical stress or strain on the device crystal to impart a preferred output.
Non-patent publications have also now started to suggest interesting arrangements of QCLs in combination with wavelength tuning performed in free space or externally with respect to the gain medium.
A description of a tunable ECQCL is presented as “Broadly tunable external cavity quantum-cascade lasers” by Maulini, R. et al. from the Institute of Physics, University of Neuchatel, Switzerland. Broadly tunable (300 cm−1) TEC cooled external cavity systems have been demonstrated. So called bound to continuum device designs support very wide gain bandwidth in these very useful systems.
Hildebrandt et al also teach of external cavity wavelength tuned QCLS. Hildebrandt points out that a QCL gain medium must be coupled to an external cavity via carefully prepared anti-reflection coatings at one device emission facet. Hildebrandt arranged his systems in a Littrow configuration to achieve selected wavelength feedback via a grating element. In another disclosure, one titled: “Quantum cascade external cavity laser systems in the mid-infrared spectral range” Hildebrandt et al similarly describe QCL gain systems coupled with external cavities. They indicate compact devices can be made via use of ZnSe collimation lenses in Littrow feedback arrangements.
Hensley, et al demonstrate on behalf of Physical Sciences, Inc. long wave (THz) QCLs in an external cavity. In a professional laboratory environment, a cryostatically cooled system is coupled on an optical bench to a movable grating via an off-axis parabolic collimation optic.
Liquid helium and significant supporting apparatus are required to realize the device. The total cavity volume is greater than 100's of cubic centimeters. The same Hensley also publishes a “QCL breath analysis system” shown on an optical bench in a collection of high precision mounting and alignment optics maintained on a rigid optical bench. Again, this system suitable for laboratory use may be characterize as another very large cavity system.
While systems and inventions of the art are designed to achieve particular goals and objectives, some of those being no less than remarkable, the art has limitations which prevent laser use in new ways now possible. Inventions of the art are not used and cannot be used to realize the advantages and objectives of the inventions taught herefollowing.